Atomic Layer Deposition (ALD) is a well known method for depositing conformal thin-films with relatively uniform thickness profile over substrates of various shapes, even over complex 3D (three dimensional) structures. In ALD the coating is grown by alternately repeating, essentially self-limiting, surface reactions between a precursor and a surface to be coated. Therefore the growth mechanism in an ALD process enables coating essentially without directional effects.
In an ALD process two or more different chemicals (precursors) are introduced to a reaction space in a sequential, alternating, manner and the chemicals adsorb on surfaces, e.g. on a substrate, inside the reaction space. The sequential, alternating, introduction of chemicals is commonly called pulsing or dosing (of chemicals). In between each chemical pulse there is commonly a purging period during which a flow of gas which does not react with the chemicals used in the process is introduced through the reaction space. This gas, often called the carrier gas or purge gas, is therefore inert towards the chemicals used in the process and purges the reaction space from e.g. surplus chemical and by-products resulting from reactions between the surface and the previous chemical pulse. This purging can be arranged also by other means, and the deposition method can be called by other names such as ALE (Atomic Layer Epitaxy), ALCVD (Atomic Layer Chemical Vapor Deposition), cyclic vapour deposition etc. The essential feature of these methods is to sequentially expose the deposition surface to precursors and to growth reactions of precursors essentially on the deposition surface. A film can be grown by an ALD process by repeating several times a pulsing sequence comprising the aforementioned pulses containing the precursor material, and the purging periods. The number of how many times this sequence, called the “ALD cycle”, is repeated depends on the targeted thickness of the film, or coating.
Many different apparatuses suitable for carrying out an ALD- or an ALD-like process are disclosed in the prior art. For example U.S. Pat. No. 6,824,816 discloses processes for depositing noble metal thin-films by ALD, and U.S. Pat. Nos. 6,174,377 and 4,389,973 describe deposition tools for ALD. A good review about the basics of ALD in general is the book; Atomic Layer Epitaxy, by T. Suntola et al., Blackie and Son Ltd., Glasgow, 1990.
The prior art discloses a wide range of materials that can be synthesized and deposited on a substrate by alternately exposing the surface of the substrate to different chemicals, in an ALD- or in an ALD-like process. But, dark colored light-absorbing oxide films being electrically conductive have not been demonstrated using ALD. For example, transparent conductive oxides (TCO), e.g. ZnO:Al and In2O3:Sn, have previously been deposited using ALD. However, even if these films of oxide are conductive, they are essentially transparent in the visible wavelength range.
U.S. Pat. No. 7,270,895 discloses an article having a layer coating with a dark color. Methods disclosed to form the coating in this publication are cathodic arc evaporation (CAE), sputtering, and PVD. A problem with these coating methods is their poor ability to uniformly and homogeneously coat non-planar surfaces and substrates with complex shapes. This is especially detrimental in decorative applications where the coating is intended to provide a specific appearance uniformly over the entire surface of the substrate.
Chromium oxide, Cr2O3, is a well known material that may exhibit dark grey colour tone. This material has been widely used and fabrication methods for chromium oxide are disclosed in e.g. U.S. Pat. No. 7,147,794. The methods for depositing chromium oxide are not able to produce films with uniform thickness and uniform optical properties over non-planar surfaces of e.g. three dimensional (3D) objects with complex shapes. Chromium and chromium oxide materials also have further drawbacks related to the allergenic properties of chromium.
The inventors have identified a need for oxide film material which is highly absorbing, i.e. dark colored even as relatively thin films, and that is electrically conductive, and a method that can form such material conformally with good thickness uniformity even over non-planar surfaces of 3D objects of various shapes.